1. Field of the Invention
The present invention relates to an optical image measurement device configured to apply a light beam to a measurement object and form an image of the surface morphology or internal morphology of the measurement object by using a reflected light or a transmitted light.
2. Description of the Related Art
In recent years, attention has been focused on an optical image measurement technology of forming an image showing the surface morphology or internal morphology of a measurement object by using a light beam from a laser light source or the like. Because this optical image measurement technology does not have invasiveness to human bodies unlike an X-ray CT device, it is particularly expected to further use this technology in the medical field.
Japanese Unexamined Patent Application Publication JP-A 11-325849 discloses an optical image measurement device having a configuration that: a measuring arm scans an object through a rotary deflection mirror (Galvano mirror); a reference mirror is disposed to a reference arm; an interferometer is used at the outlet so that the intensity of light appearing from interference of light fluxes from the measuring arm and the reference arm is analyzed by a spectrometer; and a device gradually changing the light flux phase of the reference light in non-continuous values is disposed to the reference arm.
The optical image measurement device of JP-A 11-325849 uses a method of so-called “Fourier Domain Optical Coherence Tomography (OCT)” based on technology of German Patent Application Publication DE4309056A1. That is to say, a beam of a low-coherence light is applied to a measurement object, the spectrum intensity distribution of a reflected light is obtained, and the obtained distribution is subjected to Fourier conversion, whereby an image of the morphology of the measurement object in a depth direction (z-direction) is formed.
Furthermore, the optical image measurement device described in JP-A 11-325849 is provided with a Galvano mirror that scans with an optical beam (signal light), whereby it is possible to form an image of a desired measurement region of a measurement object. Because this optical image measurement device is configured to scan with a light beam only in one direction (x-direction) orthogonal to the z-direction, a formed image is a 2-dimensional cross-sectional image of the depth direction (z-direction) along the light beam scanning direction (x-direction).
Besides, Japanese Unexamined Patent Application Publication JP-A 2003-543 discloses a configuration in which the aforementioned optical image measurement device is applied to the field of ophthalmology.
An optical image measurement device forms an image by measuring a depth almost the same as the length of a reference arm (the optical path length of a reference light). Therefore, in order to capture an image of a desired depth position, it is necessary to place a reference mirror at a position corresponding to the depth position. Considering the use of a low-coherence light, alignment of the reference mirror, namely, alignment of the measurement position in the depth direction must be conducted precisely.
Further, in an optical image measurement device, the measurement sensitivity is the most favorable at a depth position that coincides with the optical path length of the reference light (origin of the z-direction), and the measurement sensitivity becomes lower as it is distant from the origin. Also from this aspect, it is understood that alignment of the measurement position in the depth direction needs precision.
The alignment of a measurement position in the depth direction is important in a case where a measurement object moves in the depth direction during measurement, such as a case where measurement of a living body is performed. That is, in a case where a measurement object moves in the depth direction during measurement, there is the fear that an image in a target depth position cannot be captured. For example, even if the alignment of a measurement position in the depth direction is performed in advance, the measurement position is displaced in the depth direction because of movement of a measurement object, so that an image in a target depth position cannot be captured. Even if an image in a target depth position can be captured, the target depth position moves away from an original point of the Z-direction and measurement sensitivity lowers thereby, so that there is the fear that only an image of low accuracy can be captured.
Further, for example, at the time of capture of images at a plurality of sites of a fundus oculi, a measurement position in the depth direction of each of the sites may be displaced because the surface of the fundus oculi (retina) is a curved face. For example, between an image captured at the center of the fundus oculi and an image captured at a site away from the center, there is a difference in measurement position in the depth direction, whereby the positions of the images within a frame are displaced from each other. This problem generally arises in not only capture of images of the fundus oculi but also capture of images of any measurement object such that measurement positions in the depth direction vary depending on sites.